Cavity resonator circuit



Aug. 9, 1960 S. A. STAMESON CAVITY RESONATOR CIRCUIT 3 Sheets-Sheet 1INVENTOR, SPIRO A. STAMESON Filed July 18, 195B ATTORNEY Filed y 1958 S.A. STAMESON CAVITY RESONATOR CIRCUIT 5 Sheets-Sheet 2 INVENTOR, $P|R0 A.STAMESON BY 9mg- ATTORNEY Aug. 9, 1960 s.'A. STAMESON 1 4 CAVITYRESONATOR CIRCUIT L Filed July 18, 1958 3 Sheets-S eat 3 A 2 576A TYPETUBE INVENTOR, SPlRO A. STAMESON BY N. NW

ATTORNEY States Aircraft Company, Culver City, Calif., a corporation ofDelaware 7 Filed July 18, 1958, s No. 750,550"

a 4 Claims. ((1330-56) This invention relates generally to cavityresonator circuits which may be employed as the input or output circuitof very high frequency, ultra high frequency or microwave amplifiersand, more particularly, to a tunable threequarter wave folded cavityresonator circuit which may be employed with a triode, tetrode orpentode electron discharge device in a manner to operate as either agrounded grid or grid driven amplifier.

In conventionalcavity resonator circuits where single quarter wavecavities cannot be adapted to operate as either the input or outputcircuit of an amplifier employing an electron discharge device, it isthe present practice to utilize either half-wave cavities or straightthree-quarter wave cavities. A straight three-quarter wave cavity is ofconsiderable length and thus its use necessitates a corresponding largeamount of space for packaging purposes. Secondly, the straightthree-quarter wave cavity requires multiple tuning adjustments whichcomplicates the mechanical construction and makes tuning much harder toimplement. The half Wa've open cavity, on the-other hand, hasdisadvantages in that insulated rods are required for tuning adjustmentsand, in addition, presents a major problem in routing direct-currentconnections, filament connections and cooling pipes to and from thetube. Normally these latter connections have to come out through an R.F.portion of the cavity which is undesirable.

It is therefore an object of the present invention to provide animproved electron discharge device amplifier incorporating a foldedthree-quarter wave resonant cavity.

Another object of the invention is to provide a resonant cavityamplifier circuit wherein the radio-frequency and direct-current pathsto and from the tube employed are separatedv Still another object of theinvention is to provide a resonant cavity amplifier circuit including agrid bypass capacitor at a high radio frequency voltage point.

A further object of the invention is to provide an electron dischargedevice amplifier which includes a folded three-quarter wave cavitycircuit which possesses'a mechanical configuration that permits the useof a single tuning mechanism.

A still further object of my invention is to provide an electrondischarge device amplifier which includes a resonant cavity composed ofthree sections, each of which possesses a characteristic impedanceadapted to minimize frequency sensitivity.

In accordance with the present invention, a folded resonant cavity isemployed, for example, as the input circuit of a grid driven R.F.amplifier. In particular, a quarter wave cavity is folded back overfirst and second quarter wave sections which together constitute a halfwave open circuit cavity that is coupled across the control grid andcathode of the tube, the first quarter wave section being immediatelyadjacent thereto. In order to optimize bandwidth, the first quarter wavesection must have a characteristic impedance which is no less than thecapacitive reactance of the tube input and a length which atet quarterwave cavity, on the other hand, is'a section of coaxial line whichpossesses a characteristic impedance that is less than one-half that ofthe first quarter wave.

section. The characteristic impedance of the second quarter wave sectionis low so as to minimize the reactive stored energy and thus optimizethe bandwidth. When tuning the cavity the length of the second quarterwave cavity need not be changed thereby obviating the need of additionaladjustment to maintain its length constant.

Lastly, the folded-back quarter wave cavity has at.

least twice the characteristic impedance of the first quarter'wavecavity and is folded back under the second quarter wave cavity wherebythe inner conductor of the second quarter wave cavity provides its outerconductor.

The impedance of this section is made high to keep frequency sensitivityto a minimum, that is, to optimize bandwidth. Further, the length ofthis section is not affected by tuning whereby it does not requirelength com pensation.

The above-mentioned and other features and objects of i this inventionand the manner of obtaining them will Fig. 3: illustrates view B--B ofthe apparatus of Fig. 1;"

Fig. 4 illustrates a view of the left extremity as viewed in thedrawing, of the device of Fig. 1; and Fig. 5 is an enlarged view of aportion of the device of Fig. 1 showing the manner in which electricalconnections are made to the tube.

Referring now to the drawings, Fig. 1 illustrates an axial sectionalview of a preferred embodiment of the devicepf the present inventionwherein the resonant cavity of the present invention is employed as thegrid input cavity of a beam power tetrode tube. Referring also to Fig.5, the disclosed radio-frequency amplifier includes a beam power tetrodetube 10 which may, for example, be of the type which is designatedA2576A,

The tube 10 possesses a heater terminal 11, a heater and cathodeterminal 12, a control grid terminal 13, a screen grid terminal :14,anda plate terminal 15-, all of which are circular and which, as shownin Fig. l, have portions cut away for illustrative purposes. rangementof the terminals 11 to 15 on the'envelope of the tube 10 shown in thedrawing is a natural and customary terminal arrangement in beam powertetrod tubes of the type illustrated.

The radio-frequency amplifier of the present invention comprises aconventional type output resonant cavity 18 and an input resonant cavity20, the configuration and operation of which is in accordance with thepresent invention. In general, the output resonant cavity 18 is coaxialwith and surrounds a portion of the input resonant cavity 20. Inparticular, an outer wall 21 of the input cavity 20 serves as the innercylindrical boundary wall and a cylindrical metallic exterior anode wall22 serves as the outer boundary wall for the output cavity 18. The tube10 is maintainedin a centrally disposed position within the outputcavity 18 by means of an annular ceramic member 30 which has both of itslateral surfaces metallized to provide a bypass capacitor to ground andwhich is fitted with a seating ring 31 which possesses a numberofmetallic springs adapted to engage the plate terminal of the tetrodetube 10. Ceramic member 30 is, in turn, held in a centrally disposed p0sition within the output cavity 18 by means of a circular; supportmember 32. In operation the plate terminal 15 is maintained at apotential of the order of 6,000 volts,

The particular ar- The coaxial output cavity 18 is tuned to resonance bymeans of a slidable apertured disc 24 which has contacting springs toprovide electrical contact with the inner wall 21 and the outer anodewall 22 of the output cavity 18. The position of the slidable apertureddisc 24 is controlled by three longitudinal members 25, which'members 25are fastened to a ring 26 which, in turn, is engaged by means of threadswith a rotatable annular member 27. A gear arrangement 28 engages a spurgear 29 which is attached to the periphery of the annular member 27 forrotating it and thus position the apertured disc 24 in the outputresonant cavity 18 thereby to determine the frequency for which it istuned. Amplified radio frequency energy is extracted from the outputcavity 18 by a. hollow conductor 34 which is coupled to the cavity 18and, in addition, capacitively coupled to the screen grid of tubethrough annular elements 35, 36, the latter of which is connected to thescreen grid terminal 14 of tube 10. Conductor 3'4 constitutes theextension of the center conductor of a coaxial transmission line 37which extends through an opening in the exterior anode cavity wall 22.The transmission line 37 has a shorted quarter wave T stub line 38located near the output from the cavity 13 whereby the conductor 34,which as specified above is hollow, provides a radio frequency space foran insulated lead 39. The lead 39 is connected to the annular element 36and hence provides means for applying an appropriate potential to thescreen grid terminal 14 of the tube 10. In the disclosed device, theoperation of the output cavity 18 is considered conventional and hencewill not be elaborated upon.

In order to describe more clearly the input cavity together with itsmanner of operation, reference is made to Figs. 1 to 4 wherein Figs. 2and 3 illustrate views AA and B-B of the axial sectional view of theapparatus shown in Fig. 1, respectively, and Fig. 4 is an end viewthereof. Referring to these figures wherein like reference numeralsdesignate like parts or elements, the input resonant cavity 20, ingeneral, comprises a first quarter wave section 40 which has anextremity coupled across the control grid terminal 13 and the cathodeand heater terminal 12 of tube 10, a second quarter wave section 42which extends from the remaining extremity of the first section 40 to acoaxial input 43 and a folded back quarter wave section 44 which isfolded back under the second quarter wave section 42 and extends fromthe coaxial input 43 to a shorting termination 45.

More particularly, the inner boundary cylindrical wall 21 of the outputcavity 18 is extended to provide the outer boundary wall for the firstand second quarter wave sections 40, 42 of the input cavity 20. Thecylindrical wall 21 butts up against a dielectric ring 46 and isexpanded as well as extended at this point by means of an annularmetallic element '48. The dielectric ring 46 serves as a support for anannular metallic member 50 which constitutes a portion of the length ofthe inner boundary wall of the quarter wave section 40. The leftextremity of the annular metallic member 50, as viewed in the drawing,which constitutes the inner boundary wall of the quarter wave section40, is expanded and extended over three arcuate portions 51 (see Fig. 2)which have a number of finger contacts that are adapted to electricallyengage the control grid terminal 13 of the tube 10. The outer boundaryWall of quarter wave section 40, on the other hand, is extended by theannular metallic element 48 which, in turn, is attached to the peripheryof rim-shaped member 49 which has three spokes 53 that extend radiallyinwards between the arcuate portions 51 to electrically engage theheater and cathode terminal 12. The right extremity of the annularmember 50, as viewed in the drawing, is adapted to receive a metalliccup-shaped member 52 which has a central aperture and which issurrounded by axially disposed finger contact strips 54 that are adaptedto make a slidable electrical contact with the annular mem- 4 ber 50. Aswill hereinafter be explained in more detail, the electrical length ofthe first quarter wave section 40 is determined by the extent to whichthe metallic cup-shaped member 52 is inserted into the annular metallicmember 50.

The inner boundary wall of the second quarter wave section 42 isprovided by a metallic cylindrical member 56 which is supported by andextends from the outer i periphery of the cup-shaped member 52 to apoint short of the coaxial input 43. At this point the cylindricalmember 56 is connected through a capacitor 57 to the center conductor ofcoaxial input 43. Capacitor 57 is provided by undercutting the outersurface of the light extremity of the tube 56, as viewed in the drawing,sliding on a Teflon sleeve 84 which has a small flange portion whichjuts up against the under-cut and sweating an annular metallic band 85about the outer surface of the sleeve 84. This metallic band 85 isconnected to the center conductor of coaxial input 43. Capacitor 57 may,for example, have a capacitance of the order of 200 micromicrofarads.Also extending from the cup-shaped member 52 is a metallic tube 53 whichcommences from the centrally disposed aperture thereof and extends thelength of the cylindrical member 56 and is centrally supported thereinby means of a dielectric disc 60. Further, the right portion of the tube58 is capacitively coupled and physically attached to a metallic tube 62through a dielectric layer 63, the latter tube '62 being supported byfinger-contacting springs 64 which are disposed around the circumferenceof a circular aperture in an appendage 66 which, in turn, is attached asviewed in the drawings to the right extremity of the metal cylinder 21.Thus, the contacting springs 64 provide a slidable support for the tube62 thereby maintaining the tube 58 and the cylindrical member 56centered within the cylindrical wall of input resonant cavity 20.

In addition to providing the inner boundary wall of the folded backquarter wave section 44, the metallic tube 58 together with tube 62provides an R.F. free entry for heater and bias leads 70 that provideR.F. free connections to the heater and control grid terminals 11, 12,respectively. More particularly, the heater lead connects to a plug 71that is received by an annular element 72 which, in turn, has a fingercontact ring that is adapted to engage the heater terminal 11 of tube10. The heater terminal 11 is by-passed to ground through a capacitanceprovided by a metallic annular element 73 that is disposed coaxiallyabout the annular element 72 and is insulated therefrom by a thin layerof dielectric material. The left extremity of the annular element 73, asviewed in the drawing, extends radially outwards to electrically engagethe heater and cathode terminal 12 of the tube 10 and to connect throughthe spokes 53 to the rim-shaped member 49 which is maintained at groundpotential. The bias lead 70, on the other hand, is connected to thecup-shaped member 52 thereby to provide a means of applying biaspotential to the control grid terminal 13 of tube 10.

In accordance with one object of the present invention, the inputresonant cavity 20 is tunable by making only one adjustment whichadjustment tunes the length of the first quarter wave section 40 withoutaffecting the second and folded back resonant sections 42, 44,respectively. To accomplish the tuning, a threaded longitudinal element75 is rota-tably mounted in the appendage 66 and extends therefrom tothe right, as viewed in the drawing parallel to the metallic tube 62.The threaded longitudinal element 75 is engaged 'by means of threads toa member 76 which is aflixed to the end portion of the metallic tube 62and to a slidable member 78. The coaxial input 43 is attached to theslidable member 78 whereupon it moves axially along the input cavity 20along with the metallic tubes 58, 62, the cylinder 56 and cup-shapedmember 52. The outer boundary wall 21 of the input cavity 20 has anaperture to accommodate the longitudinal 82 is attached to the endportion oflthe threaded rotatable longitudinal element 75 to enable itto be turned by an a propriate amount to tune the input cavity 20.

In operation, radio frequency energy is fed through the center conductorof the coaxial input 43- and through the capacitor 57 to the rightextremity of the metal cylinder 56, as viewedin the drawing.Thistexcites both the folded back resonant quarter wave section 44together with the second quarter wave section 42. In that the foldedback quarter wave section 44 has a shorting termination 45, it presentsa very high impedance at its input which is at the output side of thecapacitor 57. The input energy is also fed to the second quarter wavesection 42 which has an inner boundary provided by the metal cylinder 56and an outer boundary provided by the cylindrical Wall 21. The secondquarter wave section 42 is terminated by the first quarter wave section40 and, as specified above, has a characteristic impedance that is lessthan one-half the characteristic impedance of the first quarter wavesection 40. Also, in order to optimize bandwidth, as specified above,the characteristic impedance of the folded back quarter wave section ismade equal to or greater than twice the characteristic impedance of thefirst quarter wave section 40. It is to be noted that the turning of thethreaded longitudinal member 75 to effect tuning of the first quarterWave section 40 does not afiect the length of the second quarter wavesection 42 or the folded back quarter wave section 44, nor the relativespacing of the coaxial input 43 with respect thereto.

Lastly, the first quarter Wave section 40 receives a signal from thesecond quarter wave section 42 and applies it across the heater andcathode terminal 12 and control grid terminal 13 of the tube 10. Asspecified above, the characteristic impedance of the first quarter Wave40, to achieve optimum bandwidth, is made greater than the capacitancereactance of the tube input.

What is claimed is:

1. A -radio-frequency amplifier comprising an electron discharge devicehaving at least a cathode terminal, a control grid terminal and a heaterterminal, an input resonant cavity including a first tunable one-quarterwave section having an extremity coupled across said control gridterminal and said cathode terminal, said first section having a firstcylindrical outer conductor and a first substantially cylindrical innerconductor disposed concentrically within said outer conductor, saidfirst inner conductor being of adjustable length thereby to enable saidfirst quarter wave section to be tuned; a second quarter wave section offixed length having an extremity coupled to the remaining extremity ofsaid first quarter wave section, said second quarter wave section havinga second substantially cylindrical outer conductor attached andelectrically connected to said first outer conductor and a second hollowcylindrical inner conductor disposed concentrically within said secondouter conductor and attached and electrically connected to said firstinner conductor; a third quarter wave section iolded back from saidsecond quarter Wave section, said third quarter wave section beingprovided by a third cylindrical inner conductor disposed concentricallywithin said second inner conductor and being attached thereto with ashorting termination one-quarter wavelength from the extremity thereofthat is common with the remaining extremity of said second section;means coupled to said second inner conductor for simultaneously excitingsaid second and third quarter wave sections with an input signal; andmeans attached to said third inner conductor for slidably moving oneextremity of said first inner conductor thereby to tune said inputresonant cavity.

2. The radio-frequency amplifier as defined in claim 1 whichadditionally includes an electrical conductor connected to said heaterterminal and extending through said third cylindrical inner conductor toa direct-current input charge device with a circular heater terminal andcon- 1 centric cathode-and gridterminals, said terminals being terminaltherebyto provide j a radio (frequency free directcu-rrentfconnection tosaid {electron discharge device. 3, Aradio-frequency amplifierhaving'anelectron disdisposed in the order named in proceeding outwards from theaxis of the evacuated; envelope of said discharge device; aninputres'onant ca'vitycomprising aifirst tunable one-quarter wave sectionhaving an extremity coupled across said grid and cathode terminals, saidfirst section having a first cylindrical outer conductor connected tosaid cathode terminal and a first substantially cylindrical innerconductor disposed concentrically within said outer conductor and whichcrosses over said outer conductor to connect to said grid terminal, saidfirst inner conductor being of adjustable length thereby to enable saidfirst quarter wave section to be tuned; a second quarter Wave section offixed length having an extremity coupled to the remaining extremity ofsaid first quarter wave section, said second quarter wave section havinga second substantially cylindrical outer conductor attached and electrically connected to said first outer conductor and a second hollowcylindrical inner conductor disposed concentrically within said secondouter conductor and attached and electrically connected to said firstinner conductor; a third quarter wave section folded back from saidsecond quarter wave section, said third quarter wave section beingprovided by a third cylindrical inner conductor disposed concentricallywithin said second inner conductor and being attached thereto with ashorting termination one-quarter wavelength from the extremity thereofthat is common with the remaining extremity of said second section;means coupled to said second inner conductor for simultaneously excitingsaid second and third quarter wave sections with an input signal; andmeans attached to said third inner conductor for slidably moving oneextremity of said first inner conductor thereby to tune said inputresonant cavity, said last-named means being rigidly attached to saidmeans coupled to said second inner conductor for simultaneously excitingsaid second and third quarter wave sections with an input signal wherebytuning said first quarter wave section does not affect tuning of saidsecond and third quarter wave sections.

4. In a radio-frequency amplifier including an electron discharge devicehaving concentric cathode and grid terminals with a predetermined inputreactance, an input resonant cavity comprising a first tunableone-quarter wave section with a characteristic impedance that is no lessthan said predetermined input reactance, one extremity of said firstsection being coupled across said grid and cathode terminals, said firstsection having a first cylindrical outer conductor and a firstsubstantially cylindrical inner conductor disposed concentrically withinsaid outer conductor and being of adjustable length thereby to enablesaid first quarter wave section to be tuned; a second quarter Wavesection of fixed length with a characteristic impedance that is no morethan one-half said predetermined input reactance, one extremity of saidsecond section being coupled to the remaining extremity of said firstquarter wave section, said second quarter wave section having a secondsubstantially cylindrical outer conductor attached and electricallyconnected to said first outer conductor and a second hollow cylindricalinner conductor disposed concentrically Within said second outerconductor and attached and electrically con nected to said first innerconductor; a third quarter wave said second section; means coupled tosaid second inner conductor for sindnltaneouslyexciting said second andthird quarter Wave sections with an input signal; and means attached tosaid third inner conductor for slidably moving one extremity of saidfirst inner conductor thereby to tune said input resonant cavity, saidlast-named means being rigidly attached to said means coupled to saidsecond inner conductor for simultaneously exciting said second and thirdquarter wave sections with an input signal whereby tuning said firstquarter Wave section does not affect tuning of said second and thirdquarter wave sections.

References Cited in the file of this patent UNITED STATES PATENTS2,706,802 Meisenheimer Apr. 19, 1959 UNITED STATES PATENT OFFICECERTIFICATE OF CORRECTION Patent N0. 2,948,858

August 9, 1960 Spir o Stameson Column 3 line 25, after "frequency"insert free Signed and sealed this 4th day of April 1961 (SEAL) Attest:ERNEST W. SWIDER XXELXHCXMME ARTHUR W. CROCKER Attesting Ofiicer ActingCommissioner of Patents

